Conventionally, different kind metal-doped conductive metal oxides such as tin-doped indium oxide (ITO), antimony-doped tin oxide (ATO), and aluminum-doped zinc oxide (AZO) have been known as materials transparent of visible light rays and having high conductivity and they have been used as transparent conductive films and transparent electrodes of liquid crystal displays and solar cells. Further, glass on which these conductive metal oxides are deposited has been used as heat ray shielding glass for windows of buildings and windows of automobiles.
Among these heat ray shielding materials, in particular, tin-doped indium oxide (ITO) is excellent in the heat ray shielding function, however since indium reserve is slight and indium is a by-product of minerals of zinc and lead, exhaustion and steep rise in costs because of mass consumption for the transparent conductive films for liquid crystal displays in recent years have been serious concern.
As a heat ray shielding material in place of ITO having these resources problem, and the supply and the cost problem, heat ray shielding materials using zinc oxide whose raw materials exist in abundance and economical have been investigated. However, zinc oxide is inferior in the shielding property of light rays with wavelength near infrared (IR) region as compared with ITO.
As heat ray shielding materials of different metal-doped zinc oxide, aluminum containing zinc oxide thin films are disclosed in Patent Document No. 1 and No. 2. However, aluminum is easier to be reacted with oxygen than zinc, a component element of zinc oxide, and as a result, in a product such as a thin film, aluminum is locally precipitated in form of an oxide and causes a problem that the optical properties such as transmission and refractive index and electric properties such as conductivity are uneven. Further, aluminum element has another problem that aluminum is not easy to be added (doped) by film formation methods other than sputtering. Furthermore, it is also difficult to add aluminum to a concentration sufficient to exhibit the heat ray shielding function.
Patent Document No. 3 discloses zinc oxide thin films containing at least one kind of Group XIV elements.
In the sunrays, IR rays having wavelength of at shortest 780 nm longer than visible light rays have a small energy quantity, about 10%, as compared with ultraviolet (UV) rays, however IR rays have a significant thermal function and are released as heat energy to increase the temperature if once being absorbed in a substance and therefore they are generally called as heat ray.
Accordingly, for example, if it is made possible to shield IR rays having the significant thermal function among sunrays coming through an aperture part, the heat shielding property is heightened and the temperature increase in the inside can be suppressed. Particularly, the energy ratio of near IR rays with wavelength of 780 to 1500 nm in IR rays is high and the energy ratio of near IR rays with wavelength of 780 to 1000 nm is particularly high. As a matter of fact, with respect to a weighed coefficient for calculating the solar transmission Ts according to JIS R 3106, it is set higher. In other words, if the near IR rays with wavelength of 780 to 1000 nm are not effectively shielded, the solar transmission can not be lowered while the visible light transmission is kept high.
However, the zinc oxide thin films containing at least one kind of Group XIV elements disclosed in Patent Document No. 3 have high transmission, about 90% of light rays with wavelength of 750 nm and about 80% of light rays with wavelength of 1000 nm and thus are incapable of sufficiently suppressing the solar transmission while keeping the visible light transmission high.
Patent Document No. 1: Japanese Kokai Publication Sho-61-96609
Patent Document No. 2: Japanese Kokai Publication Hei-1-201021
Patent Document No. 3: Japanese Patent No. 1802011